This invention relates to electronic ballasts for gas discharge lamp loads and more particularly to a ballast which permits lamp dimming, has a high power factor and is inexpensive.
Electronic ballasting circuits for fluorescent and similar gas discharge lamps are well known. Most electronic ballasts operate lamps at a fixed output level generally similar to the level which may be obtained from the lamp when operated from normal line frequency ballast circuits. The advantage in such application is obtained from the higher lamp efficiency at frequencies significantly higher than normal power frequencies of 50 or 60 Hz. High frequency operation can therefore provide equal light output with less power input to the lamp than is possible at line frequency. Alternatively, it is possible to obtain greater light output with the same electrical power input. If suitably low losses can be achieved in the high frequency ballast circuit, significant energy savings can result, particularly in the case of fluorescent lamps where the reduced system energy can result in system energy savings of greater than 20%.
Electronic ballasts which also permit dimming or control of gas discharge lamp output, are also known. The ability to control lamp output, coupled with the inherently more efficient lamp operation possible at high frequencies, can provide very significant energy savings when applied with suitable automatic controls. The savings possible with such dimmable high frequency gas discharge lamp ballasts can easily exceed 50% compared to uncontrolled line frequency ballasted systems.
Energy savings of this magnitude make such systems very desirable, yet they still have not gone into significant commercial use as compared to the number of lamps that continue to use uncontrolled line frequency ballasts. The reasons for this include: cost, susceptibility to damage due to miswire errors or accidental turn on of control devices, complex magnetic structures, poor power factor and limited control range.
High cost is a significant reason which has inhibited the introduction of electonic dimming ballasts. Line frequency ballasts have been manufactured for nearly 50 years and are highly optimized from a cost standpoint. Further, fewer individual components are needed in line frequency ballasts compared to existing frequency ballasts. Therefore, the high frequency electronic ballast has been inherently more costly than the line frequency ballast.
The large number of components and their susceptibility to damage reduces the reliability of electronic ballasts. Line frequency ballasts are better able to withstand extraordinary stresses which may be applied, particularly if a miswire condition occurs during installation. While a line frequency ballast can withstand a shorted output for many minutes, electronic ballasts usually fail immediately under the same conditions. This not only reduces system reliability, but also makes it difficult to meet safety requirements, such as those specified by Underwriters Laboratories.
Many prior art ballasts use two or more power semiconductor devices in their inverter circuits. Since these devices often dissipate significant amounts of power, they run at elevated temperatures, which substantially reduces their reliability. Also, these devices are relatively costly, both to purchase and to mount in the unit. It is obviously desirable to minimize the number of such devices to obtain maximum reliability and lower costs. Often, the use of two such devices introduces the possibility of instantaneous catastrophic failure modes if, for example, both devices were to conduct current simultaneously, instead of alternately, as is usually intended. Since it is quite possible for electronic noise, or other unanticipated events, to momentarily initiate simultaneous conduction, this represents a further reliability risk in circuits using multiple power devices.
Another common shortcoming of prior art electronic ballasts is the use of many and/or complex magnetic structures. These are needed to improve deficiencies such as poor power factor, or to reduce the total number of electronic components required. While these results are desirable, the introduction of complex magnetic components reduces the manufacturability of the unit. Although the components themselves are smaller at high frequencies, their small size requires greater precision in manufacturing. The high frequencies at which they are used often require the use of special materials and core shapes, all of which make the unit more difficult to manufacture and therefore more expensive.
An additional common shortcoming of prior art electronic ballasts is poor power factor. This is due to the use of a heavy filtered full wave bridge power supply using a large electrolytic filter capacitor to provide a d-c voltage to the high frequency inverter found in all such ballasts. Heavily filtered supplies of this type draw a relatively large capacitive current from the power line, so that in many cases, a branch circuit which can supply 90 fluorescent lamps using line frequency ballasts, may be limited to less than 70 lamps using high frequency ballasts. The filter capacitors are also physically large compared to the other components and carry relatively high ripple currents. Therefore they increase the size and cost of the unit. Also, because of their electrolytic construction and significant power dissipation, they reduce the overall reliability of the system.
In the case of electronic dimming ballasts, additional input wires are needed to provide control information to the ballast and this complicates installation. These control leads are invariably referenced to the a-c line power input circuit. Many lighting systems are connected to three-phase a-c circuits, and it is common practice to supply adjacent fixtures from dissimilar phases to reduce the perceived flicker level in the discharge lamp output. The control wires which are galvanically referenced to the a-c power input are commonly connected in parallel to cause all ballasts to follow the same control voltage. All ballasts having such parallel control leads must then be operated from the same a-c voltage phase to avoid a phase-to-phase short circuit of the a-c source through the ballasts and their parallel control leads, which would cause the destruction of the ballasts. Avoiding this condition results in additional complications in installing wiring for such a lighting system, whereby minor wiring mistakes can result in widespread destruction of ballast units.
Also, in a dimming ballast, it is common practice to use a variable voltage level on the control inputs to provide the ballast with information regarding the desired level of light output. Since the ballast is a high frequency power supply, it can easily induce noise components into the control leads which disturb the operation of the lamps. This is particularly true if the control voltage is near zero volts when the lamps are at their lowest output setting. Under these conditions, the lamps are most sensitive to disturbances and any noise picked up on the control leads is most significant when compared to the relatively low control voltage at low light output levels.
Finally, most prior art dimming ballasts are limited in their control range and vary the light output of their lamps over a ratio of only 10 to 1, or even less. While this is adequate for energy management applications, it is sufficient for most kinesthetic or architectural dimming applications where a range of 100 to 1 is more commonly required. The additional flexibility provided by such a wide range control capability significantly increases the functional and marketable value of the high frequency unit compared to the standard uncontrolled line frequency ballast. Thus, since the high frequency ballast is inherently more costly, it is necessary to offer significant performance advantages to justify the unavoidably higher unit cost.
Several prior art references demonstrate many of the above points. U.S. Pat. No. 4,414,491 dated Nov. 8, 1983 discloses a non-dimming electronic ballast with relatively few components, but using two power semiconductor devices, a large filter capacitor and a relatively complex magnetic structure. Further, the magnetic structure shown is built of relatively expensive material.
A similar device is disclosed in U.S. Pat. No. 4,392,087 dated July 5, 1983 which uses two power semiconductor devices, complex magnetics and a large filter capacitor. Dimming is obtained by voltage reduction or pulse width modulation of the power devices, but the dimming range cannot exceed a 10 to 1 ratio with standard fluorescent lamps.
U.S. Pat. No. 4,358,716 dated Nov. 9, 1982 discloses an electronic ballast which can be dimmed by duty cycle control of high frequency pulse bursts for gas discharge lamps. This unit also includes two power devices and a large filter capacitor previously discussed.
The circuit shown in U.S. Pat. No. 4,392,086 dated July 5, 1983 improves power factor by removing the large capacitor to a small auxiliary supply which is used to keep the lamp arc struck during periods of 60 Hz line power cycle when the voltage is too low for proper lamp operation. However, two power devices and several magnetic structures are used and the dimming control range is relatively limited.
U.S. Pat. No. 4,277,728 dated July 7, 1981 also uses a relatively small d-c filter capacitor in combination with an active switching regulator to improve power factor. Three semiconductor power devices are used and a large number of complex magnetic elements is required to implement the circuit. A control range of only 10 to 1 is available.
In all of the preceding prior art patents which teach dimming, the dimming control leads would be galvanically referenced to the a-c power leads. This gives rise to the aforementioned wiring complications and the possibility of catastrophic miswire conditions.
U.S. Pat. Nos. 3,619,716 and 3,731,142, assigned to Lutron electronics Co., Inc. and U.S. Pat. No. 3,265,930 disclose a single power switching device in an electronic ballast. In particular, U.S. Pat. Nos. 3,619,716 and 3,731,142 teach control of gas discharge lamps by use of a single power switching device and a pulse forming network connected across the lamp. By keeping the "on" time of the power device short compared to the lamp arc time constant, lamp current runaway is avoided and the pulse forming network stored energy is allowed to circulate through the arc when the power switch is in the "off" state. The use of a single switch eliminates the cost and reliability problems described above, and the stored energy in the pulse forming network allows wide range dimming control. These principles were applied commercially in 1974 in an electronic dimming ballast sold by Lutron Electronics Co., Inc. under the trademark "Hi-Lume".
The Hi-Lume ballast circuit uses a sample output inductor as a pulse forming network, and a current sensing resistor is placed in series with the lamp arc current. The control circuitry rectifies and filters the voltage dropped across the current sensing resistor, which is proportional to lamp arc current. This value is compared to the dimming control voltage input, and the duty cycle of the single power switching device (a switching transistor) is adjusted until the lamp arc current is stable at the level commanded by the magnitude of the dimming control voltage. A large filter capacitor supplies smooth d-c voltage to the inverter portion of the circuit. The use of an accurate servo feedback loop which directly monitors the lamp arc current results in very stable dimming capability for the Hi-Lume circuit over a range in excess of 100 to 1 light output ratio, with no striations or flicker effects visible in the lamps.
Today, 10 years after its introduction, the Hi-Lume system is still the best quality fluorescent dimming control commercially available. However, to achieve this level of performance, some compromises were required. Thus, the large filter capacitor causes a relatively poor power factor and branch circuit lamp capacities must be derated. The control circuitry is relatively complex in order to stabilize the internal servo loop and results in high cost, even though a single power device and one relatively simple magnetic structure is required. Inherent reliability is very good due to the single power device structure, but miswiring the lamp leads can result in failures since they are directly connected to the power switch and control circuitry. Also, the dimming control leads are referenced directly to the a-c line through the bridge rectifier. This means increased costs since isolation amplifier circuitry and current surge reduction circuitry must be used in conjunction with any significant numbers of these devices.